Bipolar current switching system

ABSTRACT

A current steering system is described in the context of a multi-plane magnetic core memory stack. The system is designed to control a given end of a set of core drive lines and includes a pair of bipolar switches for each pair of lines. Each bipolar switch includes two diode pairs, each pair being connected in series through a common unipolar switch to form two switchable unidirectional current paths for each bipolar switch. One current path from each bipolar switch is connected between respective ones of their associated pair of drive lines and a switchable current source of a given polarity. The remaining two current paths of the pair of bipolar switches are connected between the same two lines and a switchable current source of the opposite polarity. In this way both bipolar switches participate in controlling the flow of current in each of a pair of drive lines.

United States Patent Wells et al.

[54] BIPOLAR CURRENT SWITCHING SYSTEM I [72] Inventors: George H. 'Wells, Santa Ana; Perry B. Persons, lrvine, both of Calif.

[73] Assignee: Technology Marketing, Incorporated,

. Santa Ana, Calif.

[22] Filed: July 15, 1970 [21] App]. No.: 55,054

[52] US. Cl. ..340/l74 TB, 340/174 LA [51] lnt.Cl. ..Gllc5/02,Gllc1l/O6 [58] Field of Search ..340/174 TB, 174 LA; 307/270 [56] References Cited UNITED STATES PATENTS 3,195,114 7/1965 Gunderson 340/174 TB 3,460,092 8/1969 Davidson et al. ..340/l74 TB 3,460,093 8/1969 Huffman ..340/l74 TB 3,551,904 12/1970 Tomaszewski et a]. ..340/174 TB Primary Emminer-Stanley M. Urynowicz, .lr. Attorney-Fowler, Knobbe & Martens [57] ABSTRACT A current steering system is described in the context of a multi-plane magnetic core memory stack. The system is designed to control a given end of a set of core drive lines and includes a pair of bipolar switches for each pair of lines. Each bipolar switch includes two diode pairs, each pair being connected in series through a common unipolar switch to form two switchable unidirectional current paths for each bipolar switch. One current path from each bipolar switch is connected between respective ones of their associated pair of drive lines and a switchable current source of a given polarity. The remaining two current paths of the. pair of bipolar switches are connected between the same two lines and a switchable current source of the opposite polarity. ln this way both bipolar switches participate in controlling the flow of current in each of a pair of drive lines.

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; MA? TENS BIPOLAR CURRENT SWITCHING SYSTEM In operating a magnetic core memory, current flow must be controlled through a large number of drive lines which are threaded through the cores ofthe memory. Typically, a magnetic core memory is comprised of a stack having several layers of cores. Each layer is called a matrix and includes an array of cores arranged in several rows and columns. One set of drive lines extends through all of the columns of cores in all of the planes of the stack. These are called the column drive lines. A similar set of drive lines is threaded through all of the rows of cores. These are called the row drive lines. At one of their ends all of the row and column drive lines are connected at their opposite ends to current switches and any desired one of the drive lines may be energized by current flowing in either one of two opposite directions by actuating the current switches at its opposite ends.

Typically, the row and also the column drive lines are connected in several groups, with each group containing an even number of drive lines. Thus, a medium size memory stack may include 64 row drive lines divided into eight groups, with the drive lines in each group being connected at one of their ends. The same arrangement is also carried out with the column drive lines. At their connected ends each group of eight lines is connected to a source of positive current through a first switch and to a source of negative current through a second switch. In this manner, a desired group of lines may be enabled to conduct current in either one of two opposite directions by actuating one of their associated pair of current switches. Which of the particular lines in an enabled group will actually conduct current will depend upon which of them will have its opposite end terminated so as to complete the electrical circuit to the current source. Thus, at their source ends the drive lines require two current switches for each group of lines so that, if the total number of line groups is N, the total number of current switches required will be 2N.

The present invention is directed to reducing the number of switches required to control the flow of current at the source ends of magnetic core memory drive lines without adversely affecting the performance of the memory. In accordance with the invention, two switches are time-shared between two groups of drive lines to control the flow of current into them in both directions. All of the time shared current switches are connected either to a pair of switchable current sources or through a common pair of current switches to a current source. Thus, a magnetic core memory stack utilizing the present invention requires only N+2 current switches at the source ends of its drive lines so that, in the case where N 8, a saving of six switches is effected. The time sharing aspect of the switch arrangement of the present invention results in an important operational advantage. Thus, if the reduction in the number of switches used were effected by simply assigning a single switch to control the flow of current through a given group of lines in both directions, the rapidly alternating reversal of current which is usually required during the operation of a magnetic core memory stack would impose very great demands on the ability of the switch to effect rapid current reversal. This problem is eliminated by assigning two switches to two groups of lines in such a manner that, during a rapid series of current reversals through either one of the group of lines, current flow is alternately controlled through the two switches, so that while one of them is conducting current, the other one may recover from its previous switching cycle.

The present invention and its advantages will be more clearly understood with reference to the following description of a preferred embodiment thereof, taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a simplified, perspective schematic diagram of a magnetic core memory matrix stack driven by a conventional switching arrangement;

FIG. 2 is a simplified schematic diagram of one of the switches illustrated in block form in FIG. 1;

FIG. 3 is a perspective schematic diagram of a possible atrangement for reducing the number of switches required to steer current through the memory stack illustrated in FIG. 1;

FIG. 4 is a similar diagram illustrating the switching arrangement of the present invention for reducing the number of switches required to steer current through a memory stack of the type illustrated in FIG. 1; and

FIG. 5 is a block diagrammatic illustration of the manner in which two of the bipolar switch pairs illustrated in FIG. 4 would be connected to control the flow of current into two pairs of drive lines.

An exemplary magnetic core memory stack 11 is illustrated in FIG. 1. It includes three core planes 11a, 11b and 110. Each of the three core planes 11a, 11b, and 11c is comprised of 16 cores arranged in four rows and four columns. Corresponding columns in the three core planes are linked by a common drive line so that a total of four column drive lines 13, 15, 17, and 19 are used. Similarly, a set of four row drive lines 21, 23, 25, and 27 are linked with the respective rows of cores in the successive core planes so that corresponding rows of cores in the successive core planes are linked by the same row drive line. In a manner well known to those skilled in the art, a given set of magnetic cores may be selected to receive information by driving current through the row and column drive lines linking them. For example, to store (or write) information in the cores 29, 31, and 33 which occupy corresponding positions in the core planes 11a, 11b and lie, current is driven coincidentally through column drive line 15 and the row drive line 25 in a given direction. To read information thus stored in the cores 29, 31, and 33, current is driven through the same drive lines 15 and 25 in the opposite direction.

The cores 29, 31, and 33 occupying corresponding positions in the memory planes 11a, 11b, and 11c are commonly referred to as a word ofthe memory. Usually, the number of core planes is larger than the three illustrated in FIG. 1 so that each word of memory includes considerably more than three cores. Similarly, the number of rows and columns in a memory plane is of the order of 32 or 64. However, the operating principles of the magnetic core memory can be explained with reference to the simple three level 4X4 stack illustrated. Normally, each core plane includes in addition to the row and column drive lines an inhibit line (not shown) which is threaded through all of the cores of the core plane. These serve to allow certain cores in a word" to remain unswitched, by driving current through the inhibit windings of the core planes in which they are held.

To steer current through the desired one of the four column drive lines 13, 15, 17, and 19 they are divided into two groups of two each and the members of each group are connected together at one of their ends as shown at 35 and 37, these ends being referred to as the source end of the drive lines. The junction point 35 is connected to a source of positive current 39 and to a source of negative current 41 through current switches 43 and 45 respectively. The junction point 37 is connected to the same pair of current sources 39 and 41 through a second pair of current switches 47 and 49.

Turning now to the opposite ends of the column drive lines 13, 15, 17, and 19, hereinafter referred to as their sink ends, the sink end of each line is connected through a pair of steering diodes and through a pair of current switches to a source of positive current and a source of negative current respectively. Thus, for example, the sink end of the column drive line 13 is connected through a first steering diode 51 and through a first current switch 53 to a source of negative current 55. The same line is also connected through a second steering diode 57 and a second current switch 59 to a source of positive current 61. In practice, the respective current sources 39 and 61 may be the same positive terminal of a current supply and the respective negative current sources 41 and 55 may be the negative terminal of the same current supply so that current flows from the terminals 39 and 61 to the terminals 41 and 55 through the column drive lines 13, 15, 17, and 19.

In addition to handling the steering of current into and out of the sink end of the first drive line 13, the current switches 53 and 59 handle the same function for the sink end of the third column drive line 17 through a pair of steering diodes 63 and 65. The second and fourth-column drive lines and 19.

are controlled at their sink ends by a second pair of current switches 67 and 69 through diode pairs Hand 73 and 75 and 77 respectively. All of the steering-diodes connected between the sink ends of the column drive lines 13, 15, 17, and 19 are connected through one of four resistors 79, 81, 83, and 85 to sources of positive and negative .voltage so as to bias the diodes against conduction unless the particular one of the current switches 53, 59, 67, and 69 to which they are connected is closed. A pair of resistors 36 and 38 connect the source ends of lines 13 and 15 and 17 and 19 to ground to prevent stray currents.

Selection of the particular column drive line and the direction of current in the selected line is determined by turning on an appropriate pair of current switches at the source and sink ends'of thedesired line. For example, if it is desired that current shall flow from the source end to the sink end of the first column drive line 13, the current switches 43 and 53 will be closed. This will close a unidirectionally conductive circuit between the positive current source 39 and the negative current source 55 through the desired drive line 13. The closing of the switches 43 and 53 is effected over their respective control input lines 87 and 89 from a control logic system 91. All of the remaining switches at both ends of the column drive lines 13, 15, 17, and 19 are controlled individually from the same control logic system 91.

The control switches are all of the unidirectionally conductive or unipolar type as exemplified by the circuit shown in FIG. 2. Basically, each of them includes a transformer 93 whose primary winding 95 is connected to a source of control signals, such as the control logic system 91 and whose secondary winding 97 is connected between the base and emitter of a switching transistor 99. The switched path is through the emitter-collector circuit of the transistor 99 and is closed when a signal is applied to the transformer primary winding 95.

The arrangement for steering current in the desired direction through a desired one of the row drive lines 21, 23, 25, and 27 is the same as that described in detail with reference to the column drive lines and need not be discussed further. It will be sufficient to observe that the switches which control the flow of current through the row drive lines are also actuated from the control logic system 91 as discussed previously. It was noted earlier that a typical magnetic memory core stack will include a much larger number of rows and columns than is shown in FIG. 1. Assuming, for example, that such a core stack has 64 rows and 64 columns, the organization shown in FIG. 1 would be expanded by dividing the 64 column drive lines into eight groups of eight each and connecting the source ends of each group of eight together to a given pair of current switches so that each current switch pair controls eight drive lines rather than two as shown in FIG. 1.

.At the sink ends of the drive lines each drive line would be connected through an individual pair of steering diodes to a given switch pair. There would be a total of eight switch pairs associated with the sink ends of the drive lines and each switch pair would control one line in each of the eight groups of eight lines. Thus, the basic method for selecting a particular line would still include selecting the group by means of the proper switch associated with that group at the source end and selecting the particular line within the group by means of the proper switch at the sink end. It will be observed that, regardless of the number of lines in a group and also regardless of the number of groups of lines, the arrangement of FIG. 1 requires two current'switches at the sink end for each group of lines, one to steer current into the source end of the line and the other to steer current in the opposite direction. Thus, in the case of a 64 by 64 matrix stack having a total of 16 groups of drive lines, with eight lines in each group, a total of 32 control switches will be required at the source ends of the drive lines alone.

One approach to reducingthe number of current switches required is illustrated in FIG. 3. The system of FIG. 3 is illustrated in the context of controlling the flow of current in the.

through respective unipolar current switches and 117.

The current source 41 and its associated switch 117 may be considered together as a switchable current source, and the same also applies to current source 39 and its associated switch 1 15. I

The two bipolar switches 111 and 113 may be of identical construction. Using the first bipolar switch 111 as an example, it is comprised of a single unipolar switch 119 having a current input terminal 121 joined to the cathodes of a pair of current steering diodes 123 and 125, and a current output terminal 127 joined to the anodes of a second pair of current steering diodes 129 and 131. The anode of the steering diode 123 is connected to the current output terminal of the unipolar switch 115 andthe cathode of the current steering diode 131 is connected to the drive line 35 so that together they form a unipolar current path from the switch 115 through the switch 119 to the line 35. Similarly, the anode of the steering diode 125 is connected to the line 35 and the cathode of the steering diode 129 is connected to the input terminal of the unipolar current switch 117 so that they together form a unipolar current path from the line 35 through the switch 119 to the switch 117. The current switch 113 includes a unipolar switch 120 and a set of four steering diodes 124, 126, 130, and 132 connected in the same manner as their corresponding components in the current switch 111.

To steer current from the source 39 into the line 35 the unipolar switches 115 and 119 are closed, causing a flow of current into the line 35 through the steering diodes 123 and 131. If, on the other hand, it is desired that current flow in the opposite direction in the drive line 35, the switch 1 17 is closed instead of the-switch 115, in which case the flow of current is through the steering diodes 125 and 129 and again through the unipolar current switch 119 between the two diodes. Thus, it is seen that the flow of current through the drive line' 35 is controlled in both directions by means of the same unipolar current switch 119. The control of current through the drive line 37 is effected by an identical set of components in the bipolar current switch 113.

If the arrangement of FIG. 3 were used with only two drive lines 35 and 37 (or only two groups of such lines), there would not be any saving over the arrangement shown in FIG. 1 in the number of switches used to drive current through them. In

both cases the number of switches required would be four. However, where the number of groups of lines is more than two, such as eight for example each pair of groups of lines will require the addition of only a single bipolar switch pair, and all of the additional bipolar switches can be fed through the same pair of unipolar current switches 115 and 117. Thus, instead of requiring sixteen unipolar current switches, the arrangement of FIG. 3 would reduce the required number to eight bipolar current switches and two unipolar current switches. This represents a significant economic advantage, since the bipolar current switch is practically the same as the unipolar current switch, with the exception of the addition of four inex- V pensive steering diodes.

The arrangement shown in FIG. 3 suffers from an important disadvantage, however. This is due to the inherent method of operation of a magnetic core memory. This operation is such that the state of a given rowof cores in the memory is often reversed repeatedly, thus requiring that the flow of current in a given drive line be reversed at short intervals. Each sequence of steering current into a particular drive line, such as the line 35, and then steering current out of that line in the opposite direction may be thought of as a switching cycle. During the first half of the cycle current flows into the line 35 and during the second half of the cycle current flows out of the same line. During both halves of the current steering cycle current must flow through the same unipolar current switch 119 of the bipolar switch 111. This places a-very heavy burden on the switch 119 and in particular on its ability to recover after the first half of its switching cycle in time to carry out the second half of its switching cycle. Thus, with reference to FIG. 2, if the transformer primary winding 95 is pulsed during the first half of the switching cycle to cause current to flow into the drive line 35, the transistor 99 must turn on and then turn off again during that first half of the switching cycle and before the beginning of the second half. This is so, because during that second half the transistor 99 will again be called upon to turn on for a short period of time so as to steer current in the opposite direction and out of the drive line 35 through the steering diodes 125 and 129. g

It will be noted that, while the unipolar current switch 119 is thus heavily driven twice during each switching cycle, its sister switch 120 in the bipolar switch 113 lies idle and is not utilized. The present invention does away with this shortcoming by so connecting the bipolar switches 1 11 and 113 that the alternately flowing and oppositely directed currents in a given one of the lines 35 and 37 are steered through alternate ones of the switches 119 and 120, thereby sharing the burden of switching equally between them. A preferred arrangement for achieving this objective is shown in FIG. 4.

The switching system shown in FIG. 4 is comprised of the same components that make up the switching system of FIG. 3. Each component in FIG. 4 is numbered the same as its corresponding component in FIG. 3 but with the suffix a added. The difference between the systems of FIG. 3 and FIG. 4 is in the manner in which their components are connected between the sources of current and the drive lines to which current is to be routed. In the system of FIG. 4, as in FIG. 3, current is steered from a positive source 39a and a negative source 41a through either of a pair of lines 35a and 37a. Interposed between the current sources 39a and 41a and the lines 35a and 37a are a pair of bipolar switches 111a and 113a each of which includes a pair of unipolar (i.e., much more conductive in one direction than in the opposite direction) current paths sharing a common switch. Thus, in the case of the bipolar current switch 111a 'there is a first unipolar current path through the diodes 123a and 131a and another current path through the diodes 125a and 129a, both current paths sharing a common switch 119a. In the case of the bipolar switch 113a one unipolar current path leads from the diode 124a to the diode 132a through the switch 120a and the second unipolar current path leads through the same switch 120a from the diode 126a to the diode 130a. Both of the switches 119a and 120a may be unipolar switches of the type shown in FIG. 2.

As in the system of FIG. 3, a pair of switches 115a and 117a serve to determine whether current is to flow from the positive source 39a or into the negative source 41a. In accordance with a feature of the invention, one unipolar current path from each of the bipolar switches 1 11a and 1 13a is connected in series with one of the switches 115a and 117a and the other unipolar current paths from the two bipolar switches 11 1a and 113a are connected in series with the other one of the switches 115a and 117a.

More specifically, as shown in FIG. 4, the switch 117a has connected in series with it a unipolar current path in the bipolar switch 111a and another unipolar current path in the switch 113a. The first current path is comprised of the diode 129a, the switch 119a, and the diode 125a and has its opposite end connected to the line 35a. The second current path is in the bipolar switch 113a, includes the diode 130a, the switch 120a and the diode 126a and has its opposite end connected to the line 370. In a similar manner, the switch 115a also has two unipolar current paths connected in series with it, one in each of the bipolar switches 111a and 113a. The first unipolar current path includes the diode 1230, the switch 119a, and the diode 131a and has its opposite end connected to the line 37a. The second unipolar current path is comprised of the diodes 124a and 132a and the switch 120a, and is connected at its opposite end to the line 35a.

To steer current from the positive current source 39 a through the line 35a, such as for reading a set of memory cores, the switches a and a will be closed concurrently in response to signals applied to them concurrently from the control logic system 91a. Current will be steered through the two switches by means of the steering diodes 124a and 132a of the bipolar switch 113a.

If now, it is desired to reverse the flow of current in the line 35a, so as to cause it to flow into the current source 41a, such as for writing into the cores, the switches 117a and 119a will be actuated so that current will be steered through them through the steering diodes a and 129a of the bipolar switch 111a. Thus, it is seen that, for a given one of the pair of lines 35a and 37a, current flowing in opposite directions will not be switched by the same bipolar switch but will instead be switched by respective ones of the two bipolar switches 111a and 113a. The same applies to current being switched to flow in the line 37a. As indicated before, this has the advantage of alternating the operation of the switches 119a and 120a during a typical core read-write sequence during which the flow of current is rapidly reversed several times through a drive line of the memory.

The reduction in the number of switches required by the system incorporating the present invention is illustrated in FIG. 5, showing a system in which two pairs of lines are serviced by two pairs of bipolar switches under the control of the control logic system 91a. The flow of current into the first pair of lines 35:11, 37a1, is controlled by a pair of bipolar switches 1 11a, 1 13a, connected in the same manner as shown in FIG. 4

in series with a pair of current switches 115a, 117a. A second pair of bipolar switches 111a, 113a is connected in series between a second pair of lines 35a2, 37112 and the same pair of current switches 115a, 117a. Additional pairs of lines may be connected in a similar manner through additional pairs of bipolar switches to the same common pair of current switches 115a, 117a, limited only by their current carrying capacity. If a conventional switching arrangement such as that shown in FIG. 1 were used, two unipolar switches would be required for each line for a total requirement of eight unipolar current switches. With the arrangement shown in FIG. 5, only six unipolar current switches are used, one in each of the bipolar switches 111a, 113a and one in each of the unipolar current switches 115a, 117a. Moreover, each additional pair of lines will require the addition of only two bipolar current switches, each of which contains only a single unipolar current switch. This is in contrast with the conventional arrangement in FIG. 1 in which the addition of each pair of lines would require an additional four unipolar current switches.

It will be appreciated that although the invention has been described in the context of a coincident current type of magnetic core memory, it could be used in other types of magnetic core memories as well, particularly where there is a need to send current in opposite directions over a pair of drive lines.

What is claimed is 1. A bipolar switching system for steering current in opposite directions from a first current source and a second current source through either of a pair of lines comprising:

a. first and second bipolar switching means, each including first and second unipolar current paths sharing a common switch;

b. first and second means for switching current;

c. means for connecting one unipolar current path from each said bipolar switching means in series with a given one of said lines so as to switch current individually between respective ones of said current sources and said given line; and

(1. means for connecting the other unipolar current path from said bipolar switching means in series with the other one of said pair of lines so as to switch current individually between respective ones of said current sources and said other line.

2. A bipolar switching system for steering current in opposite directions from a first and second selectively switchable source through either of a pair of lines comprising:

a. first and second bipolar switching means, each including 7 l a single unipolar switching means,

2. first and second unidirectionally conductive means a connected to said single switching means to define in combination therewith first and second individual, switchable, unipolar current paths;

b. means for connecting one unipolar current path from each said bipolar switching means for individually switching current between one of said lines and respective ones of said first and second current sources; and

c. means for connecting the other unipolar current paths from said bipolar switching means for individually switching current between the other one of said lines and respective ones of said first and second current sources;

d. whereby'current is steered through each of said lines in opposite directions by respective ones of said unipolar switching means.

3. A bipolar switching system for steering current from a positive source and a negative source through either of a pair of magnetic core memory drive lines comprising:

a. first ans second means for switching current;

b. first and second bipolar switching means, each including 1 a transistor connected to operate as a switch,

2. first and second pairs of steering diodes, the diodes in each pair being connected in series with one another through the emitter-collector circuit of said transistor to define in combination therewith first and second individual switchable, unipolar current paths;

c. means for connecting one switchable unipolar current path from each said bipolar switching means in series with one of. said drive lines for individually switching current between said one of said drive lines and respective ones of said sources; and

d. means for connecting the other switchable unipolar current paths from said bipolar switching means in series with the other one'of said drive lines for individually switching current between said other one of said drive lines and respective ones of said sources.

4. In a magnetic core memory, a switching system for steering current from a selectively switchable positive source and a selectively switchable negative source through a selected one of a plurality of drive line pairs of said memory comprising:

' a. a pair of bipolar switching means associated with each said line pair, each member of each said pair of bipolar switching means including two unipolar current paths sharing a common unipolar switch; 1

. b. means associated with each said pair of bipolar switching means for individually connecting one unipolar current path from each of the respective members of said pair in series between one of the lines with which said pair of bipolar switching means is associated and respective ones of said positive and negative current sources;

c. means associated with each said pair of bipolar switching means for individually connecting the other unipolar current path from each of said respective members in series between the other one of said lines with which said pair of bipolar switching means is associated and respective ones of said positive and negative current sources; and d. means for selectively switching one of said sources and one of said unipolar common switches concurrently. 5. A system for steering current in opposite directions through a pair of magnetic core memory drive lines comprising: a. first and second bipolar switching means, each including first and second paths for current flowing in respectively opposite directions; and b. means for connecting the first path of said first bipolar switching means and the second path of said second bipolar switching means to one of said magnetic core drive lines, and means for connecting the second path of said first bipolar switching means and the first path of said second bipolar switching means to the other of said magnetic core drive lines so that current is steered in opposite directions in said drive lines by switching from one to the other of said bipolar switches, said first bipolar switch steering current in one direction through one of said drive lines and in the said second direction through the other of said drive lines and second bipolar switch steering current in respectively opposite directions through said pair of drive lines.

' Y r a a: 

1. A bipolar switching system for steering current in opposite directions from a first current source and a second current source through either of a pair of lines comprising: a. first and second bipolar switching means, each including first and second unipolar current paths sharing a common switch; b. first and second means for switching current; c. means for connecting one unipolar current path from each said bipolar switching means in series with a given one of said lines so as to switch current individually between respective ones of said current sources and said given line; and d. means for connecting the other unipolar current path from said bipolar switching means in series with the other one of said pair of lines so as to switch current individually between respective ones of said current sources and said other line.
 2. first and second unidirectionally conductive means connected to said single switching means to define in combination therewith first and second individual, switchable, unipolar current paths; b. means for connecting one unipolar current path from each said bipolar switching means for individually switching current between one of said lines and respective ones of said first and second current sources; and c. means for connecting the other unipolar current paths from said bipolar switching means for individually switching current between the other one of said lines and respective ones of said first and second current sources; d. whereby current is steered through each of said lines in opposite directions by respective ones of said unipolar switching means.
 2. first and second pairs of steering diodes, the diodes in each pair being connected in series with one another through the emitter-collector circuit of said transistor to define in combination therewith first and second individual switchable, unipolar current paths; c. means for connecting one switchable unipolar current path from each said bipolar switching means in series with one of said drive lines for individually switching current between said one of said drive lines and respective ones of said sources; and d. means for connecting the other switchable unipolar current paths from said bipolar switching means in series with the other one of said drive lines for individually switching current between said other one of said drive lines and respective ones of said sources.
 2. A bipolar switching systEm for steering current in opposite directions from a first and second selectively switchable source through either of a pair of lines comprising: a. first and second bipolar switching means, each including
 3. A bipolar switching system for steering current from a positive source and a negative source through either of a pair of magnetic core memory drive lines comprising: a. first ans second means for switching current; b. first and second bipolar switching means, each including
 4. In a magnetic core memory, a switching system for steering current from a selectively switchable positive source and a selectively switchable negative source through a selected one of a plurality of drive line pairs of said memory comprising: a. a pair of bipolar switching means associated with each said line pair, each member of each said pair of bipolar switching means including two unipolar current paths sharing a common unipolar switch; b. means associated with each said pair of bipolar switching means for individually connecting one unipolar current path from each of the respective members of said pair in series between one of the lines with which said pair of bipolar switching means is associated and respective ones of said positive and negative current sources; c. means associated with each said pair of bipolar switching means for individually connecting the other unipolar current path from each of said respective members in series between the other one of said lines with which said pair of bipolar switching means is associated and respective ones of said positive and negative current sources; and d. means for selectively switching one of said sources and one of said unipolar common switches concurrently.
 5. A system for steering current in opposite directions through a pair of magnetic core memory drive lines comprising: a. first and second bipolar switching means, each including first and second paths for current flowing in respectively opposite directions; and b. means for connecting the first path of said first bipolar switching means and the second path of said second bipolar switching means to one of said magnetic core drive lines, and means for connecting the second path of said first bipolar switching means and the first path of said second bipolar switching mEans to the other of said magnetic core drive lines so that current is steered in opposite directions in said drive lines by switching from one to the other of said bipolar switches, said first bipolar switch steering current in one direction through one of said drive lines and in the said second direction through the other of said drive lines and second bipolar switch steering current in respectively opposite directions through said pair of drive lines. 